Pilot symbols are needed in OFDM communication systems in order for the receiver to undertake channel estimation. Known methods of multiplexing the pilot symbols with the data symbols include time-multiplexing (i.e. allocating certain symbols to pilot signals rather than data), frequency-multiplexing (i.e. using only certain sub-carriers for pilot signals, and other sub-carriers for data), and pilot superimposition (i.e. adding a pilot signal to the data and transmitting the combination of pilot and data in the same symbols, where the pilot signal is selected to facilitate separation from the data signal).
Superimposed pilots have the advantage of allowing the pilot signals to be spread throughout the time-frequency space occupied by the signal, thus facilitating channel estimation in high-Doppler or rapidly-varying channels.
In static channel conditions, channel estimation using superimposed pilots is more complex to implement than using time- or frequency-multiplexed pilots. Consequently, in some systems such as the Long-Term Evolution of UMTS, it is desirable to combine the use of time-multiplexed pilot symbols with the possibility to use superimposed pilots using a common structure.